Case reports suggest that lead stored in the skeleton is mobilized during pregnancy and lactation, placing the mother and, in particular, the developing fetus at risk of lead poisoning. The placenta is fully permeable to lead. To investigate the hypothesis that skeletal lead is mobilized during pregnancy and lactation, we propose a prospective cohort study with repeated measures of blood, plasma and bone lead in an ethnically diverse population of young, inner city women. We hypothesize that as lead is mobilized during pregnancy and lactation, maternal and fetal blood lead levels will increase and maternal bone lead levels will decrease. We will compare bone, plasma and whole blood lead levels prior to pregnancy with measurements obtained from the same women in the immediate postpartum period and six months after delivery. We hypothesize that bone lead levels will decrease and blood lead levels will increase as a result of lead mobilization during pregnancy. Among women who breast feed, the six month postpartum measurements will assess any additional effect of lactation on lead mobilization. Similar comparisons will also be made in a control group of non-pregnant women. The control group will allow us to compare the natural history of bone lead changes in the absence of pregnancy and lactation. Umbilical cord blood lead levels will be determined to assess maternal transfer of circulating lead to the neonate. This study population is expected to have a high body lead burden as a result of environmental lead exposure. The ethnic diversity of this population will enable us to determine whether body lead burdens are higher in African-American and Hispanic women than in Caucasian women. We will also assess whether women with higher body lead burden at baseline mobilize more lead than women with lower baseline body lead burden. Adequate iron stores have been shown to protect against lead absorption and therefore, possibly protect against lead mobilization during pregnancy. This study will investigate the effect of iron stores on body burden of lead in two ways. First, we will determine whether higher iron stores, assessed by serum ferritin levels, predict a lower body burden of lead. Secondly, we will determine whether the level of iron stores affects lead mobilization during pregnancy by comparing serum ferritin levels with bone, blood and plasma lead measurements before and after pregnancy. This study will provide the first quantitative examination of the hypothesis that skeletal lead mobilization occurs during pregnancy and lactation. The investigation should provide important insights into the potential health effects of chronic environmental lead exposure to pregnant women, developing fetuses and infants through breast feeding.